I tried the Ultra Thrust on the SK .91 and it didn't seem to do much with the 15-6. Could be it is tuned for rpm not in the range of this engine. I have switched back to the stock muffler.

I finally got around to swapping out my engines and have now flown the second SK .91. Same results. Runs great. It's had about a half gallon through it on the bench and I can tell it's a little tighter. It seems to run better with a slightly richer setting. This will probably go away like it did on the first engine.

I am trying a few different props, all 15-6, but different brands. I have an APC, Master Airscrew, Master Airscrew Scimitar and Zinger. I had been flying the Scimitar on the first engine, but had run the black MA on the second on the bench. When I got the APC in, I put it on to fly the newer engine. I did not see anythin startling about the APC. I then tried the MA. It is noisier, but my buddy, Flaps, says it looks to him like it performs better. I need to do some more testing with all of them as I fly in engine #2. I just love testing out new engines, but we went seaplane flying today and I didn't get to fly the Ultra Stick.